Having written Haskell for over a year now for personal projects, understanding the memory model can be one of the hardest aspects of Haskell, which can make it frustrating to write allocation-free code (although some techniques like deforestation are done by the compiler to eliminate intermediate structures entirely).

Linear types being added in GHC 8.12 would be a big deal because it would allow programmers to be able to write allocation-free code that can use mutable data structures with a pure API (as opposed to the ST state monad), much like how Rust solves this with the ownership system.